The Volga-Urals basin is one of the largest oil-producing regions in Russia. The Orenburg region, an essential part of the Volga-Urals basin, presents most of the challenges found in the other parts of the basin. More than half of the wells produce exclusively from carbonate formations and require periodic acidizing treatments to maintain economical production. Carbonate stimulation treatments are usually based on pumping of hydrochloric acid and specialized diverters into the formation to maximize zonal coverage. Some of the formation properties in the basin create additional challenges for successful acidizing treatment, including significant reservoir pressure depletion, high water cut, formation heterogeneity, low reservoir temperature, crude with sludge tendency and large thief zones created by previous acidizing treatments. In many cases, uncertain formation properties make the task even more complex.

Carbonate stimulation practices in the Orenburg region have continuously improved over the last 6 years. It started with the implementation of degradable balls and polymer-based self-diverting acid to improve zone coverage. Later, viscoelastic, self-diverting acid was introduced to eliminate retained damage associated with polymers. At the same time, viscoelastic selective diverter was used to minimize the risk of water cut increase after the treatment. Finally, foam diversion with and without coiled tubing placement was implemented to account for reservoir pressure depletion. Many lessons were acknowledged during this period with recommendations for better technologies utilization at different reservoir and wellbore conditions. Apart from technologies, another essential part of carbonate acidizing success was proper laboratory testing. A comprehensive Quality-Assurance Quality-Control (QAQC) standard has been developed and implemented for acidizing treatments. Moreover, additional laboratory work, such as core flow testing or proppant embedment tests, has been done to clarify some additional aspects of performed treatments.

The majority of the fields in Orenburg region are mature. This implies that quality of candidate wells degraded from year to year. However, continuous improvements in stimulation practices and rigorous quality control helped maintain production at economic rates_and even increase the scope of work compared with previous years. We describe experience gained during 6 years of acidizing treatments in the Orenburg region, including lessons learned and best practices.